Implementing a Dispose method
【Implementing a Dispose method】
0、Dispose() and Dispose(Boolean)
The IDisposable interface requires the implementation of a single parameterless method, Dispose. However, the dispose pattern requires two Dispose
methods to be implemented:
A public non-virtual (
NonInheritable
in Visual Basic) IDisposable.Dispose implementation that has no parameters.A protected virtual (
Overridable
in Visual Basic)Dispose
method whose signature is:
1、Dispose() has a standard implementation, The Dispose
method performs all object cleanup, so the garbage collector no longer needs to call the objects' Object.Finalizeoverride.
实现System.IDsiposable.Dispose()方法。不能将此方法设置为virtual,子类不能override此方法。此方法的实现必须为如下代码。
// Implement IDisposable.
// Do not make this method virtual.
// A derived class should not be able to override this method.
public void Dispose()
{
Dispose(true);
// This object will be cleaned up by the Dispose method.
// Therefore, you should call GC.SupressFinalize to
// take this object off the finalization queue
// and prevent finalization code for this object
// from executing a second time.
GC.SuppressFinalize(this);
}
2、实现析构函数(Finalize)。析构函数必须实现为以下代码。
// Use C# destructor syntax for finalization code.
// This destructor will run only if the Dispose method
// does not get called.
// It gives your base class the opportunity to finalize.
// Do not provide destructors in types derived from this class.
~MyResource()
{
// Do not re-create Dispose clean-up code here.
// Calling Dispose(false) is optimal in terms of
// readability and maintainability.
Dispose(false);
}
3、实现virtual Dispose(bool disposing)方法,必须为protected。如果dispoing为true,说明从Dipose()方法调用而来;如果为false,说明从析构函数(Finalize)调用而来。在析构函数中,不能再使用引用类型(Ref)成员类型,所以只处理unmanaged resource的释放即可;而在Dipose()调用过程中,managed res & unmanaged res都要释放。
最后,添加一个成员变量disposed来标记是否已经释放过。
// Dispose(bool disposing) executes in two distinct scenarios.
// If disposing equals true, the method has been called directly
// or indirectly by a user's code. Managed and unmanaged resources
// can be disposed.
// If disposing equals false, the method has been called by the
// runtime from inside the finalizer and you should not reference
// other objects. Only unmanaged resources can be disposed.
protected virtual void Dispose(bool disposing)
{
// Check to see if Dispose has already been called.
if(!this.disposed)
{
// If disposing equals true, dispose all managed
// and unmanaged resources.
if(disposing)
{
// Dispose managed resources.
component.Dispose();
} // Call the appropriate methods to clean up
// unmanaged resources here.
// If disposing is false,
// only the following code is executed.
CloseHandle(handle);
handle = IntPtr.Zero; // Note disposing has been done.
disposed = true; }
}
3.1、Here's the general pattern for implementing the dispose pattern for a derived class that overrides Object.Finalize:
A class derived from a class that implements the IDisposable interface shouldn't implement IDisposable, because the base class implementation of IDisposable.Dispose is inherited by its derived classes.
using System; class DerivedClass : BaseClass
{
// Flag: Has Dispose already been called?
bool disposed = false; // Protected implementation of Dispose pattern.
protected override void Dispose(bool disposing)
{
if (disposed)
return; if (disposing) {
// Free any other managed objects here.
//
} // Free any unmanaged objects here.
//
disposed = true; // Call the base class implementation.
base.Dispose(disposing);
} ~DerivedClass()
{
Dispose(false);
}
}
4、An object must also call the Dispose method of its base class if the base class implements IDisposable.
如果基类有Dispose()方法,子类必须的Dispose()必须调用基类的Dispose()。
5、什么时候需要Dispose()?
1)包含Disposable对象的容器类需要实现Dipose,而不需要实现Finalize()。因为清空一个容器,并不表明需要释放unmanged resource。
2)自身包含unmanged resource的类需实现Dispose、Finalize。
6、DO throw an ObjectDisposedException from any member that cannot be used after the object has been disposed of.
当调用一个已经Disposed对象时,抛出 ObjectDisposedException。
public class DisposableResourceHolder : IDisposable {
bool disposed = false;
SafeHandle resource; // handle to a resource public void DoSomething(){
if(disposed) throw new ObjectDisposedException(...);
// now call some native methods using the resource
...
}
protected virtual void Dispose(bool disposing){
if(disposed) return;
// cleanup
...
disposed = true;
}
}
7、一些C#自带类库的Close()语言与Dispose()语义是一样的。
public class Stream : IDisposable {
IDisposable.Dispose(){
Close();
}
public void Close(){
Dispose(true);
GC.SuppressFinalize(this);
}
}
参考:
1、https://docs.microsoft.com/en-us/dotnet/api/system.idisposable.dispose?view=netframework-4.7.1
2、https://docs.microsoft.com/en-us/dotnet/standard/design-guidelines/dispose-pattern
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